SUMMARYTubal eggs, uterine blastocysts, and ectoplacental cones from C57BL inbred matings were transplanted to the kidney, and blastocysts to the pseudopregnant uterus, of hyperimmune C3H mice. The ectoplacental cones and those blastocysts transferred to the uterus developed normally whereas the tubal eggs and blastocysts transplanted to the kidney all failed to develop. These results are interpreted as demonstrating that immunological reactions are directed against the histocompatibility antigens present in the eggs transplanted to the kidney. Normal development of blastocysts in the uterus indicates a protective function of the surrounding decidual tissue. It is suggested that fibrinoid material which develops around each trophoblast cell protects the ectoplacental cone from immunological attack.